One of the applications to which the present invention relates concerns magnetic elements that are used to detect magnetic fields on the basis of measuring the extraordinary Hall effect.
There are two types of Hall effect: the ordinary Hall effect and the extraordinary Hall effect. The ordinary Hall effect occurs in metallic materials or doped semiconductors and is due to the Lorentz force that acts on electrons under the influence of a magnetic field. The extraordinary Hall effect occurs to a limited extent in ferromagnetic materials and is the result of diffusion of electrons due to spin-orbit interaction with the magnetic moments of the ferromagnetic material.
One characteristic parameter of the Hall effect is the Hall resistivity which is given by the following equation:ρxy=(Vxy/I)t=R0H+4πRsMz  (1)where:                Vxy is the Hall voltage measured in the plane of the thin film in a direction perpendicular to that of the electric current,        I is the intensity of the current that flows in the plane of the thin film,        t is the thickness of the thin film,        R0 is the ordinary Hall coefficient,        H is the amplitude of the applied magnetic field,        Rs is the extraordinary Hall coefficient and Mz is the perpendicular component of the magnetisation of the thin film.        
The first term of equation (1), R0H, corresponds to the ordinary Hall resistivity and the second term, 4πRsMz, corresponds to the extraordinary Hall resistivity. For relatively weak magnetic fields, the ordinary Hall effect is generally several orders of magnitude smaller than the extraordinary Hall effect and can therefore be ignored.
If the magnetisation of the ferromagnetic film is parallel to the plane, which is generally the case for thin films, its perpendicular component Mz, increases linearly with the applied out-of-plane magnetic field until saturation magnetisation Ms is reached. Therefore, as long as Mz is less than Ms the extraordinary Hall voltage is proportional to the applied magnetic field.
FIG. 1 schematically shows variation in Hall resistivity as a function of the applied magnetic field for a thin-film magnetic material whose magnetisation is parallel to the plane. For H<4πMs, resistivity varies linearly with the applied field until ρxy=4πRsMs is reached. Beyond this point, resistivity varies linearly with the applied field with a slope R0 (first term of equation (1)) which is much smaller, as indicated previously. The useful operating region of a magnetic field sensor based on this principle is therefore confined to magnetic field values less than 4πMs, where Ms is the saturation magnetisation of the magnetic material in question.
It is the slope of the ρxy(H) slope that determines the magnetic field sensitivity of such a sensor, expressed in microhms centimeter per tesla (μΩcm/T). This slope can also, taking into account the thickness of the thin film t and depending on the relation ρxy=t×Rxy, be expressed in ohms per tesla (Ω/T). In order to maximize this slope, one can either increase the extraordinary Hall effect term Rs or reduce the planar magnetic anisotropy of the material in order to reduce the perpendicular saturation field.
It is useful to choose a material with high longitudinal resistivity and high spin-orbit diffusion in order to maximize Rs. Such high longitudinal resistivity is also an advantage because, for small-sized devices, it makes it possible to preserve a sufficient voltage response whilst limiting the current that flows in said device to values that can be withstood, below the value that will cause irreversible structural modifications (electromigration phenomena).
High longitudinal resistivity is obtained either by increasing the atomic disorder of the material (a disordered alloy for example) or by reducing the thickness of the film (it is known that, for an extremely thin film, electrical resistivity varies approximately inversely to the thickness of the film). The contribution made by spin-orbit diffusion is augmented by choosing a material that contains elements with a high atomic number, platinum, palladium, gold or metals in the lanthanide series for instance.
It should nevertheless be noted that the incorporation of such materials must necessarily be limited in terms of their concentration in order to preserve the satisfactory magnetic properties of the ferromagnetic alloy.
The second way of increasing the slope of the ρxy(H) curve is to reduce the planar magnetic anisotropy of the material, i.e. introduce an additional term of opposite sign to the conventional term 4πMs. The origin of this term denoted by HS can be volume anisotropy of magnetocrystalline origin, volume anisotropy induced by elastic growth strains or interface anisotropy due to interfacial electronic interactions. The influence of a layer of platinum in contact with a magnetic layer of cobalt, nickel or iron is a typical case, for example.
When this additional term is present, perpendicular anisotropy field HK can be expressed as follows:HK=4πMs−HS  (2)
Qualitatively, perpendicular anisotropy field HK will therefore reduce uniformly as HS increases, magnetisation of the magnetic layer always being parallel to the plane until it approaches zero, the limit beyond which, when HS exceeds 4πMs, magnetisation of the magnetic layer will spontaneously (i.e. without any applied magnetic field) be perpendicular to the layer plane. In this latter case, the magnetic material has two stable states in a zero field and can therefore be integrated into devices such as magnetic memories or magnetic logic gates.
FIG. 2 is a schematic view of the magnetisation curve obtained in this situation for a typical sample having the following composition: Pt1.8 nm/Co0.6 nm/Pt1.8 nm. This curve can be obtained either by traditional magnetometric measurement, by measuring the Hall effect, by measuring the Kerr effect or the Faraday magneto-optical effect.
The arrows show the direction of travel of the magnetic cycle when excitation field H indicated on the x-axis is applied. The unidirectional arrows represent irreversible cycle changes, whereas the bidirectional arrows represent reversible cycle changes. Magnetisation levels are given in arbitrary units on the y-axis.
By gradually reducing the applied magnetic field from a positive value, for example, to a zero value, magnetisation of the magnetic layer remains perpendicular to its plane and oriented in the direction of the previously applied magnetic field. It is then necessary to apply a magnetic field of opposite direction having a more or less large amplitude in order to re-orientate magnetisation of the layer, still in a direction perpendicular to its plane, but in an opposite direction, namely in the direction of this new applied magnetic field.
The magnetic field required in order to obtain this change, more precisely the magnetic field that must be applied in order for magnetisation, averaged over all the magnetic domains, to be zero is referred to as the coercive field and is denoted by Hc in FIG. 2. The value of this coercive field will determine magnetisation stability in one direction or the other in the event of exposure to external magnetic disturbances.
The stronger this coercive field, the more stable the material is when exposed to such disturbances. This also, however, makes it more difficult to deliberately modify the magnetisation direction by applying a perpendicular magnetic field, such as, for instance, when the material is used as a magnetic memory in which information is in fact coded by the magnetisation direction.
A weak coercive field that requires less energy to change magnetisation may therefore be preferable, with the possible need to magnetically “shield” the device against external magnetic disturbances or a strong coercive field may be preferred because it makes the device more stable (but consumes more energy when information is written to the device).
It should also be mentioned that one can use other ways of determining the orientation and amplitude of the magnetisation of a magnetic layer, for example magneto-optical effects whereby interaction between incident light and the magnetic moments of the magnetic layer cause rotation of the polarisation plane of the incident light and thus modification of its axial ratio.
To determine the orientation of the magnetisation of this magnetic layer, one can also add a second magnetic layer that is separated from the first layer by a layer of a non-magnetic metal or by an oxide layer with the direction of the magnetisation of this second layer being known. By measuring the electrical resistance of such a trilayer structure, one can determine the direction of the magnetisation of the first magnetic layer since the electrical resistance of this stack is smaller if both magnetisations are parallel compared with when magnetisations are antiparallel (the well-known giant or tunnel magnetoresistance phenomenon). Electric current may flow either in the plane of the layers or in a direction that is perpendicular to the layer plane if the sample has been cut into a post having lateral dimensions of the order of 1 μm or less.
To determine the orientation of the magnetisation of this magnetic layer, one can also position, close to the layer, a magnetoresistive read head that will be affected by the field radiated by the magnetic domains of this layer
The literature contains examples of several materials that may have some of the properties explained above.
Examples include well-known cobalt/platinum multilayer systems that have high perpendicular magnetic anisotropy. Depending on the thicknesses of the elementary platinum and cobalt layers, the number of (Co/Pt) pattern repetitions and the presence of a platinum buffer layer, one can obtain a system having either magnetisation that is spontaneously (in a zero magnetic field) perpendicular to the plane of the layers or magnetisation in the layer plane but with extremely high perpendicular magnetic susceptibility (slope of curve MZ=f(H)). Unfortunately, these materials have low longitudinal resistivity due to the low resistivity of their metallic components.
Structures similar to those in the present invention have also been studied (see B. Rodmacq, S. Auffret, B. Dieny, S. Monso, P. Boyer, “Crossovers from in-plane to perpendicular anisotropy in magnetic tunnel junctions as a function of the barrier degree of oxidation”, Journal of Applied Physics, 2003, Vol. 93, p. 7513). They consist in stacking, on a substrate, successive layers of platinum, cobalt and alumina or reverse stacking one of the stacks proposed in this invention.
The amplitude of the Hall effect in these structures is relatively small, mainly due to the presence of a relatively thick platinum layer into which a non-negligible portion of the electric current that flows in the structure is diverted.
Stacked layers similar to those of the present invention have already been proposed in the literature, but not with a view to applications in magnetic field sensor, magnetic memory or magnetic logic component type devices.
Firstly, one can cite work on multilayers composed of cobalt layers alternating with alumina layers (Al2O3) (see Ch. Morawe, H. Zabel, “Structure and thermal stability of sputtered metal/oxide multilayers: the case of Co/Al2O3”, Journal of Applied Physics, 1995, Vol. 77, p. 1969). The authors of this publication do not mention any perpendicular magnetic anisotropy property of these materials which is the subject of the present invention.
In addition, in contrast to the present invention, the authors do not intend these materials for use in the area of magnetic field sensors or magnetic memories or logic components based, in particular, on measuring the extraordinary Hall effect. In fact, the authors suggest using these materials only because of their structural properties in the context of mirrors for x-rays. This work falls outside the application area referred to by the present invention.
One can also cite work on “discontinuous” multilayers comprising layers of cobalt or cobalt-iron alloys alternating with layers of silicon dioxide SiO2 or alumina Al2O3 (see B. Dieny, S. Sankar, M. R. McCartney, D. J. Smith, P. Bayle-Guillemaud, A. E. Berkowitz, “Spin-dependent tunnelling in discontinuous metal/insulator multilayers”, Journal of Magnetism and Magnetic Materials, 1998, Vol. 185, p. 283). The authors use the term “discontinuous” to refer to the fact that the layers of cobalt or cobalt-iron alloy are not in the form of thin films of virtually uniform thickness but rather in the form of cobalt or cobalt-iron aggregates coated in the oxide matrix.
These systems are prepared by cathode sputtering by alternately depositing layers of magnetic metal and oxide layers. The atoms of cobalt or cobalt-iron tend to coalesce in the form of discontinuous blobs which result in a planar structure of more or less independent aggregates, depending on the thickness of the deposited metal embedded in the isolating matrix.
In these structures, only a “memory” of this thin layer of cobalt or cobalt-iron is retained and this situation is completely different to that considered in the present invention. No mention is made of any perpendicular magnetic anisotropy properties of these materials in these studies relating to discontinuous metal/insulator multilayers. Finally, the authors do not intend these materials for use in the area of magnetic field sensors or magnetic memories or logic components based, in particular, on measuring the extraordinary Hall effect. This work therefore falls outside the application area referred to by the present invention.
One can also cite work dealing with multilayers fabricated by depositing a cobalt-iron alloy followed by natural oxidation of the surface of this alloy in the presence of oxygen (see G. S. D. Beach, A. E. Berkowitz, “Co—Fe metal/native-oxide multilayers: a new direction in soft magnetic thin film design I. Quasi-static properties and dynamic response”, IEEE Transactions on Magnetics, 2005, Vol. 41, p. 2043, and G. S. D. Beach, A. E. Berkowitz, “Co—Fe metal/native-oxide multilayers: a new direction in soft magnetic thin film design II. Microscopic characteristics and interactions”, IEEE Transactions on Magnetics, 2005, Vol. 41, p. 2053).
These multilayers have high electrical resistivity, a strong magnetic moment and considerable magnetic “softness” (ease with which magnetisation can be saturated in a direction parallel to the layer plane). The authors do not mention any special property in respect of the amplitude of the extraordinary Hall effect.
Although they present results that appear to indicate a tendency for a certain perpendicular anisotropy to occur for thin magnetic metal thicknesses, more detailed scrutiny of their results shows that the observed reduction in the amplitude of perpendicular anisotropy field HK is essentially due to a decrease in the term 4πMs (see equation (2)) rather than to any contribution made by perpendicular anisotropy term HS.
Similarly, the authors predict that magnetisation will spontaneously be perpendicular to the layer plane for magnetic metal thicknesses less than 1.1 nm, but, at the same time, present results for a thickness of 1.0 nm, i.e. below this limit, which nevertheless unambiguously show that, in this case, magnetisation is parallel to the layer plane and not perpendicular.
Finally, one can cite work dealing with stacks comprising two multilayers (Pt/Co) and (Co/Pt) separated by a NiO oxide layer (see C. Christides and Th. Speliotis, “Polarity of anomalous Hall effect hysteresis loops in (Pt/Co)15/AF/(Co/Pt)15 (AF=FeMn, NiO) multilayers with perpendicular anisotropy”, Journal of Applied Physics, 2005, Vol. 97, p. 013901).
In this document, it appears that the two multilayers either side of the NiO layer have their magnetisation perpendicular to the layer plane in the absence of the NiO oxide layer. This is clearly apparent when comparing FIG. 2 (NiO layer present) and FIG. 3 (NiO layer absent), page 013901-3; these Figures show that, in both cases, the magnetisation of the magnetic layers is oriented at right angles to the layer plane. In contrast to the present invention, it is therefore not the presence of the separating NiO layer that lends this stack its perpendicular magnetic anisotropy properties.
The above considerations demonstrate that no currently known magnetic material combines all the necessary properties for use as a magnetic field sensor or magnetic memory, namely high longitudinal resistivity (at least several hundred μΩcm), high Hall resistivity (several percent of the longitudinal resistivity) and high perpendicular magnetic susceptibility (at least ten times higher than that of a conventional magnetic material with planar magnetisation, with an out-of-plane saturation field of the order of several dozen milliteslas), or even perpendicular magnetisation in a zero field for magnetic memory or magnetic logic gate type applications.
Moreover, for magnetic memory or magnetic logic gate type applications, known materials of the (Pt/Co) type or lanthanide series/transition metal alloys do not have all the required properties for this type of application (high electron spin polarisation (if magnetoresistance is used to detect the signal), good thermal stability during annealing, low corrosion).